Blog 7: Week of 12/03/2018-12/08/2018: for this week, using the guest lecture on regenerating of the cardiovascular vessel and the final class of review on decellularization towards tissue engineering various organs, I will discuss how can we potentially employ optimized decellularization tissue engineering for the application of potentially regenerating patients own organs such as heart and kidney. This final blog will tie my earlier discussion on Tissue Engineering to finally discuss the possibilities of tissue engineering in the future.

Decellularization is a process used in Tissue Engineering to separate the extracellular matrix (ECM) of tissue of the most important organs such as liver, heart, lung, kidney, and skin from its inhabiting cells which potentially leaves the ECM scaffolds (platforms) of these organ tissues, that can be potentially employed in artificial organ and tissue regeneration with patients original cells. Decellularization can potentially revolutionalize how we deal with organ regeneration in tissue engineering as organ failure is a major problem in medicine.

Figure 1: Organ failure is a major issue in medicine and organ transplantation is the method used to mitigate the issue and yet even after organ transplant patients have immunoreaction complications.

Just in the United States, over 34,770 organ transplants were performed in 2017 and over 114,000 people are on the national organ transplant waiting list (1). In interpreting this, one can ask, why decellularization in the first place, why not just transplant the original organs or tissue to a patient? Well, the answer to this question can be surprisingly broad, but for simplification purpose, we can rewind what has been done to help patients with organ or tissue failure in the past. Some of the methods including transplant of organs from donor humans or other animals yet this method have a potential drawback in that the recipient immune system attacks the introduced (new parts) into the body (transplant rejection). Consequently, the recipient patient immune system has to be suppressed to avoid transplant rejection. Again suppressing the immune system costs the patient in that the patient will be easily exposed for infection or potentially for cancer. Therefore, a better method (tissue engineering) has to be employed if the goal is to help the patient in a long course. The early tissue engineering methods aim to artificially recreate organs or tissue in vitro using biodegradable materials, then implant these organs to a patient. While the idea sounds marvelous, it again introduces a complication in actually creating organs that closely mimic the physiological organs of the patients because the cells in each organ get its structure and function from a sophisticated feedback system in its environment in which all of this dictated by the environment the cells found itself. Most importantly, these in vitro formed organs mechanically fails and potentially introduces immune reaction from the body. Therefore, what’s next in tissue engineering if the ultimate goal is to regenerate the patient’s own organ or tissue?

Figure 2: Decellularization tissue engineering of four major organs. The decellularized organs will be seeded with patients own cells to potentially regenerate the patient’s own organ

This where decellularization comes in, decellularization tissue engineering acknowledges that the cells in each organ differentiated and perform their function by integrating the environmental signal they’re in: the environment dictates the cellular differentiation, migrations, and communications. The idea is then where do we get this environment that dictates to recreate organs or tissue? The answer is to use already existed organs from a donor or from another animal, then to remove all the cells (thus eliminating immune reaction and organ rejections) and seed the new environment with the patients own stem cells so that they can differentiate to regenerate patients own tissue or an organ. Now, in decellularizing the original organ, a precise or careful method has to employed in order to optimize the amount of material is removed from that organ because most of the current methods have issues in accomplishing this. That’s if highly decellularized, we would lose potential ECM components or if decellularized too light, we would run into an immune reaction and potentially organ rejection. Therefore there is a need to optimize this method depending on which organ or tissue need to be regenerated. To better understand, which component of the ECM needs to be abundant to specifically dictate the differentiation of cells into the desired organ, we need to understand the most important component of ECM as discussed below.

Figure 3: The extracellular matrix (ECM) contains varies components which play a critical role in dictating the cellular functions in each organ and keeping most of this component in optimized decellularization is a key step in regenerating patient’s own organs.

The extracellular matrix is composed of [1] structural proteins such as collagen and elastin, [2] specialized proteins such as fibronectin families proteins and laminin and finally [3] proteoglycans such as Hyaluronic acid (HA) and chondroitin sulfate.
collagens are structural proteins with various forms and types and abundant in the extracellular matrix. They can be found from bovine, porcine, rat and humans. The main functions of collagens in additions of structural proteins include facilitation of cell adhesion, cell migration, cell survival, and cell proliferation. It’s formed through a series of postmodification of a protein. First, a collagen fibril is formed from the ribosomal translation these peptides will be cleaved into small units with propeptide cleavage. Then these small fragments of collagen proteins will be cross-linked with lysyl oxidase cross-linking which gives the protein to have a triple alpha helical structure (2). Further, these modifications give collagen to have a Toe region in stress-strain mechanical test graph. By zooming in to one of the many functions of collagen are its adhesive sites and its functions. The multiple adhesive sites play a critical role in cell signaling with receptor-specific domains. Among these adhesives, RGD which as special signals for the cells that bind in that signal to spread out over time.

Elastin is another class of structural proteins which gives tissue its elasticity, the ability to stretch and bounce back to its original state (3). It’s mostly found in tissues and skins of the body. In the connective tissues it enables the tissue to resume their shape after stretching or contracting and in the skins it helps the skin flexible yet tight and smooth as it stretches to accommodate normal activities such flexing a muscle or opening and closing or our eyes. Elastins are created by fibroblasts which are also connective tissues and as we age, the capacities of fibroblasts decreasing thus losing elasticity in skins and tissues the highly utilize elastin. One of the potential characteristics of elastin is that once elastin is last, it’s difficult to regain it. The damaged can because from overexposure to sunlight or from hormonal stress and smoking and the main one being aging.

Laminins are the fundamental structural component of a basement membrane which interacts with a receptor on epithelial and endothelial cells to determine behavioral responses (4). Laminin plays a major role in influencing cell differentiation, adhesion, and migrations, particularly in wound healing. Glycosaminoglycans (GAGs) family of glycoproteins also plays a critical role in absorbing water to modulate the mechanical properties of the ECM.

Figure 4: The future of decellularization tissue engineering is a promising field to potentially recreate patients own organ and tissue such heart. Decellularization have several steps and each step have to be carefully optimized to achieve a desired outcome in the patient.

In summary, there are the different type of ECM competes that are need to be carefully steadied and kept during decellularization to specifically regenerate particular organs such as the heart. That’s to say different organs have a different composition of each ECM component to perform its function. For example, if the goal is to regenerate a skin, then the decellularization method employed should leave a higher proportion of elastin because the skin needs to be elastic to retain its original shape during contraction and relaxing. Further, these ECM components are not there just for the mechanical purpose, they also dictate cellular differentiation, migration, and cellular adhesion during cellular signal transductions. Therefore, tissue engineering is a promising field and the next step of tissue engineering is to optimize the decellularization method to effectively regenerate personalized organs for each patient.

Sources:

1. Organ Donation Statistics | Organ Donor. (n.d.). Retrieved from https://www.organdonor.gov/statistics-stories/statistics.html
2. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 22.3, Collagen: The Fibrous Proteins of the Matrix. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21582/
3. Organ Donation Statistics. (n.d.). Retrieved from https://www.organdonor.gov/statistics-stories/statistics.html
4. Peternugraha Follow. (2018, October 28). Laminin 5: Roles & utility in wound healing. Retrieved from https://www.slideshare.net/peternugraha/laminin-5-roles-utility-in-wound-healing?qid=265bb72b-d9cb-47a6-b79c-1f4d6feb9421&v=&b=&from_search=1

Blog 6: Week of 11/26/2018-11/30/2018: for this week, using lecture fourteen [metals and their applications], I will discuss how can we potentially employ tissue engineering in bone and vascular remodeling using metals and techniques related in tunning mechanical properties.

Could Metals be integrated into Tissue Engineering? Before directly addressing the proposal, it crucial to present the current use of metals and issues associated with the various metal incorporation in the body.  When it comes to mechanical strength, metals outperforms most materials that are available with a few exceptions and not to mention how they revolutionalize the construction industry. Wherever around us, most of the building incorporate some forms of metals mostly steel in structural support and related functions.

Figure 1: Osteoarthritis is a major issue in which structural supporting bone tissue such as cartilage is unable to perform their function due wearing out over time

Now the question becomes how can we take advantage of these mechanical strengths to integrate them for biomedical setting such structural support to mitigate problems in patients with osteoarthritis and related problem? Just to highlight some critical health problems that are related to structural support problem includes osteoarthritis, a condition in which the structural support and lubricant cells such as cartilages are unable to perform their physiological functions due to varies causes the main one being worn out over time.

Figure 2: Osteoarthritis is increasingly becoming a major issue as population ages

The prevalence of the disease increases as the aging population increases, just between 2013-15, over 54.4 million adults have been diagnosed with arthritis (1). 
Further, finding biocompatible metals with similar mechanical properties of bone such Mg is an active area of research for tissue engineering to solve cardiovascular problems such as atherosclerosis.

Metals have been used for structural support such as titanium alloy to alleviate these problems due to their mechanical strength. Despite their theoretical implications, when these metals introduce into the body, they cause unintended problems such activation of an inflammatory response.

Figure 3: bone is under constant remodeling to accommodate the change in patients body weight and to maintain blood Ca2+ and phosphate ions. 

Above all, due to their mechanical strength, these metals have been mostly incorporated into structural support; a substitution for the bone. The problem is that the bone is constantly being remodeled in the body to compensate the patients by wait change. Further, the bone is the main source of Ca2+ and phosphate ion which are the two most essential ions in the body which involve in a single cascade mechanism including in the central nervous system, in muscle nerve connection and hormonal signaling cascade and much more (2). That’s when the body needs these ions, the bone will resorption via osteoclasts to release these ions into the bloodstream. When the patient’s weight increase, on the other hand, the osteoblasts bone cells will absorb these ions back into the bone matrix and make the bone dense or strong to support the body. But when the natural bone is substitute with a metal, these natural feedback system will be disturbed. The bone no longer remodels because the weight of the patient is shielded by the metal.

Furthermore, biomaterial reduction in the body another issue: metals causes an inflammatory response which can potentially lead to significant predicament to the surrounding tissue. Currently, researchers are working on how to take advantage of mechanical strength while reducing the above major issue including biocompatibility, that’s by tunning the properties of metals. Next, let’s highlight some of the breakthroughs in metallurgical modulation to that increase progress of tissue engineering.

The first issue of the metals is matching or closely resembling the mechanical properties such as the strength of the biomaterials in the body. Metals strength can be altered with a series mechanism to match the required specifications. For instance, if the metal we’re using has low compressive strength than the design specification, we can employ cold working process, that’s by heating the metal below its melting point and using different mechanisms such as rolling or drawing.

Figure 4: Cold working such as rolling enables reduce grain size thus limiting grain dislocation which enables to increase the strength of the metal

This mechanism potentially reduces the grain size of the given metal thus increasing boundaries between grains, and restraining the dislocation of the grains in the metals. Consequently, the metal strength will increase substantially. What if we want to reduce or correct the strength of the given metal to match again to specification? Again, we can use a countering mechanism: employ the annealing method, a heat treatment that reduces the mechanical strength at high temperature using three steps: recovery, recrystallization and grain growth. In short, the metal can be remelted and the rate of nucleation, the grain size will be controlled, thus controlling the strength of the metals to even with specifications.

The next problem is corrosion of the metal in the body. Here, you may suggest why not just use metals that have corrosion resistant and high mechanical strengths. Well, the environmental energy dictates what reaction happens in that environment: what I’m trying to say is that in the outside world where oxygen is abundant, iron has high reaction affinity with oxygen (due to the difference in reduction potentials), iron will corrode while aluminum seems not to corrode. But now we’re interested in integrating metals in the body, completely a different environment. The body is not at equilibrium rather it’s in steady state condition in which it’s constantly catalyzing redox reaction to maintaining the body at optimal conditions such as 37 degrees Celsius, PH of 7.4,…that means this new environment in the body changes the chemistry and can potentially corrode a metal easily which drives a material failure in the body (3). How can these problems be resolved? First, the standard reduction potential of each metal is carefully studied and metals with different characters will be selected to fit in that new environment. That’s some metals may improve to resist corrosion for body’s environment and yet they may not mechanical properties we’re looking like strength, thus we can take strong metals such as titanium alloys, and we can coat them with a metal with low surface chemistry within the body, that way we can eliminate the corrosion problem.

Figure 5: HA is associate in signal transduction for bone regeneration

Another issue is that biocompatible, that’s we want our metal based biomaterial not only just used as a structural support but want to also modulate the surrounding tissue to survive and perform its physiological function. This issue can be lessened by surface coating metals with bioactive materials such Hyaluronic acid (HA), which can potentially increase bioactivity such as osteointegration, a process that enables to integrate the sounding bone cells into inserted biomaterial so that these bone cells can perform their physiological functions, like growth, absorption of ions, resorption of ions (4). Further, metals such Magnesium is becoming an active area of research as they are biocompatible and has similar mechanical properties as the bone.

Putting all of these techniques together, we can potentially build biomaterials of composites of metals and other bioactive materials to potentially closely mimic the physiological function of the bone and other parts in the body. This domain is an active area of research to potentially transform tissue engineering, a branch of biomedical engineering that can operate with aim of substituting biological parts in tissue and organ level. That includes highly prevalent disease such as remodeling blood vessel, heart, kidney, pancreas and other tissue or organs by directly taking the patients to stem cells which make them their biocompatible and eliminating the need immunosuppress drugs.

Sources

1. National Statistics | Data and Statistics | Arthritis | CDC. (n.d.). Retrieved from https://www.cdc.gov/arthritis/data_statistics/national-statistics.html
2. Home. (n.d.). Retrieved from https://americanbonehealth.org/nutrition/how-the-body-maintains-calcium-levels/
3. What Is Normal Body Temperature? (n.d.). Retrieved from https://www.webmd.com/first-aid/normal-body-temperature#1
4. What is Hyaluronic Acid? How does Hyaluronic Acid benefit the body? (n.d.). Retrieved from https://www.hyalogic.com/about-ha/about-hyaluronic-acid/

Blog 5: Week of 11/05/2018-11/10/2018: for this week, using lecture Twelve [Bulk Metalic Glass (BMGs)], I will discuss how can we potentially employ tissue engineering in blood glucose meter sensor and stent by increasing a surface area of biosensors or biomaterials  for dynamic blood glucose level readout and robust blood vessel regeneration using BMGs.

Diabetes Mellitus is a global pandemic with over 388 million people affected worldwide (1). In addition to its direct consequences on patients daily life impediments, it’s also associated in doubling the risk of having a heart attack.

Figure 1: the global prevalence of diabetes.

Type II diabetes accounts about 90% of all global diabetes cases and 80% of these cases are believed to be preventable by changing diet and lifestyle such as physical activities (2). And in order to accomplish these lifestyle goals, blood glucose meter (biosensors) play an important role.
However, the current glucose meter’s performance has been impeded with its biosensors ability in the body to interact with blood glucose. That’s despite designing an incredible device and inserting into the patient, the patient develops immunological reaction and prevents the glucose meter from performing its functions because the sensor will be blocked from interacting with blood glucose in flat sensor due to the biomaterials biochemistry interaction within the patient’s body.

Figure 2: Blood Glucose Meter. This type of blood glucose sensor requires multiple injections with the patient’s tissue which cause pains and distress. BMGs sensors, on the other hand, can be implemented in the patient’s body to measure dynamic blood glucose level, eliminating the need to poke a patient multiple times.

To further expand on this mechanism, let me discuss how the cells first interact with biomaterial inserted in tissue.  One of the ways the cells react to biomaterials is that when biomaterials are introduced into a tissue in the body, proteins will have adhered into the biomaterial, then the macrophage interrogation will occur causing a big cell surrounding of the biomaterials. Eventually, the formation of envelope around the biomaterials increases inflammation and most importantly the biomaterials will have less integration (contact) with the surrounding cells which reduce the main purpose the biomaterials were inserted in the first place.  You might ask, what is the problem with this process? Well,  for instance, to dynamically measure the blood glucose level of a patient, a blood glucose meter (sensors on the glucose meter) needs to make a close contact with the surrounding cells so that the sensor will read out the blood glucose level. Further, the contact surface area needs to be large without increasing the device. Thus, to effectively monitor a blood glucose level, we have to overcome several obstacles from the biomaterial standpoint and from the immunological standpoint.

From biomaterials standpoint, how can we design the glucose sensor to have a more surface (contact) area while reducing its immunological inflammation (making it more biocompatible) without increasing its size? This is where the BMGs come in; by making a nanoscale division increment, it’s possible to achieve large surface while reducing immunological inflammations.

Figure 3: BMGs can be designed to have such large surface area in nanoscale which was hard to achieve such performance with other types of biomaterials. A large surface on biosensors enables to perform robust and dynamic blood glucose level readout.

BMGs has combined properties and functions between metals and plastics that enables it to achieve such promises. That’s plastics are easy to work with low temperatures but they’re brittle, and metals have the high mechanical strength yet they’re hard to work with at low temperature (challenging to process in a laboratory setting due to their grain boundaries). BMGs, on the other hand, can be used to make nanoscale biomaterials such as glucose meter sensors without being broken, which is very difficult to achieve such scales using ceramics and just metals because ceramics and metals have crystals (with boundaries of grains) and  as the resolution gets smaller into nanoscale sensors, the grain boundaries in these materials (metals and ceramics) causes microscale mechanical failure (3). But the BMGs don’t have these grain boundaries, and so we can design nanoscale biosensors.

Figure 4: fracture crack in crystals due to grain boundaries which impedes ceramics refining abilities like BMGs. BMGs resolves fracture crack because they lack such big grain boundaries.

And the more surface area sensor has more contact with the blood sugar, outputting robust and accurate dynamic blood glucose level readout we can get. Therefore, BMGs enables to design and insert a glucose meter sensor into the patient’s body that can dynamically control the glucose level of a diabetes patient and encourages the patient to take immediate actions to better their health such as doing exercises.
The mechanical strength, easily processability, its corrosion resistance in combination with its biocompatibility, BMG is a  promising biomaterial in designing and implementing tissue engineered stent for robust regeneration of blood vessels in the cardiovascular system. Particularly, porous BMG is promising for stent design and tissue engineered cells that can integrate and replace the biomaterials overtime.

Figure 5: BMGs have promising abilities to be incorporated in stent design due to their mechanical strength, porosity (for cell integration & tissue regeneration) and corrosion resistance.

Moreover,  BMGs have the mechanical strength of a metal to overcome a blood vessel shear stress without being corrosive and toxic for the surrounding cells.

Sources:

1. (1) “Managing Diabetes Mellitus With Cell Therapy.” Villa Medica, 11 May 2018, villa-medica.com/cell-therapy-for-diabetes/.
2. (2) “Physical Activity.” American Diabetes Association, www.diabetes.org/are-you-at-risk/lower-your-risk/activity.html.
3. (3) Qazi, Meelu. “Chapter 8 Mechanical Failure.” LinkedIn SlideShare, 3 Dec. 2013, www.slideshare.net/MeeluQazi/chapter-8-mechanical-failure.

Blog 4: Week of 10/21/2018-10/27/2018: for this week, using lecture ten, I will discuss how can we potentially employ tissue engineering for bone regeneration using the bioactive biomaterials discussed. 

The endoskeletal system is an essential system that facilitates movement,  load-bearing role, and providing the structural maintenance and for the protection our most vital and delicate organs such as the brain and heart.

Figure 1: the human endoskeleton system consisted of 206 bone that provides structural support, organ protection, ion homeostasis, and site of immunological cell regeneration and maturation.

In addition to these primary structural supports, it serves a key function in Ca and P homeostasis, and function as the site of immunological cells generation (the bone marrow for hematocrit regeneration). The homeostasis of Ca and P play a significant role in signal activation or inactivation throughout the various muscle types in the bone. Consequently having a misfunction of a bone tissue induces huge complications in one life..causing one to lose one or more of the above inherent life functions of the bone tissue.

In order to alleviate these issues, biomaterials have been employed in different hallmarks.  Nevertheless, the proposed implantation of these biomaterials in the past mostly has been in the form of a bio-inert material(1). And so, the contemporary consideration biomaterials to incorporate into tissue engineering methods, toward a design that employs bioactive materials that will integrate with biological cells for the regeneration of the bone tissue. Before addressing how tissue engineering could be employed for bone tissue regeneration, it’s relevant to discuss the fundamental organizations regarding the bone tissue.

The bone is made of bone matrix and the cells that made it. The bone matrix consists of osteoid, an organic material which includes various proteins and mainly collagen type I which dictates the tensile strength of the bone. The second constituent of bone matrix is hydroxyapatite, an inorganic mineral which comprises calcium phosphate crystals which dictates the rigidity and density of bones.

Figure 2: bone cell types that synthesis the bone matrix which serves as the framework for bone regeneration.

The cells that made matrix can be grouped into three main groups. The osteoprogenitor cells which are the precursor for the osteoblast cells which growth factors. The first types of cell that build the matrix are the osteoblast cells, which builds (synthesis) the two matrix components which are the primary structures for bone tissue regenerations.  These osteoblast cells then matured towards osteocytes which occupy space in the matrix with sensors for bone cellular communication (are considered mechanosensory cells of the bone). And the last types of bone cells are osteoclasts that crash bones matrix to release Ca and P ions which play in reabsorption of bone.

The osteoblasts build a bone while the osteoclasts crash the bone. Thus the bone is under a constant remodeling homeostasis. The implication of the remodeling of the bone is that it keeps the homeostasis of Ca and P ions in the bone. When calcium is needed in the body for signal activation or inhibition, the osteoclasts crashes bone to release these ions and when the body has high Ca and P ions, the osteoblast take up these ions to build the bone.

So, knowing the physiological regeneration of bone tissue, how can we use biomaterials to regenerate bone tissues in order to substitute degraded bones? Given the high demand of clinical need of biomaterials for orthopedic, we  can also tailor the biomaterials to be osteoinductive (able to support the differential of progenitor cells to an osteoblast lineage),  osteoconductive (cable to promote bone growth and encourage the ingrowth encompassing bone) and finally osseointegration (cable of integrate within the surrounding bone) (2).

Figure 3: Hydroxyapatite, the major consist of the bone tissue matrix and a major candiate of biomaterirals in bone tissue engineering.

Therefore,  biomaterials are required to be bioactive materials (including bioactive ceramics, bioactive glasses, and or a combination with polymers) to be incorporated in bone tissue engineering. Using these bioactive materials, the scaffold (framework) will be built in a way that over time, these materials will be biodegraded slowly and replaced with the patient’s own newly regenerated bone tissues.

Generally speaking, the bioactive inorganic materials that encompass tricalcium phosphate, bioglasses, and their combination with polymers can be tailored to design varies characteristics of a specific scaffold. Once scaffold is designed with these bioactive and biodegradable materials, various growth factors will be delivered to the scaffold using nanoparticle in order to enhance the regeneration of bone tissues as shown below (3).

Figure 4: different mechanism of introducing growth factors in the bone matrix to facilitate bone tissue regeneration

Finally, in modeling the bone tissue engineering, if we can master the modeling of biological complexity with three-dimensionally synthetics bioactive materials and the and closely mimic the physiological communication complexity of cells in the matrix, we can potentially expedite the field of bone tissue engineering to regenerate patient own bone tissue that will solve the incompatibility issue including immunological  reactions due to the introduction of bio-incompatible materials in the body.

Sources:

1. Lanza, Robert, Robert Langer, and Joseph P. Vacanti, eds. Principles of tissue engineering. Academic press, 2011.
2. Stevens, Molly M. “Biomaterials for bone tissue engineering.” Materials today 11.5 (2008): 18-25.
3. Lee, Soo-Hong, and Heungsoo Shin. “Matrices and scaffolds for delivery of bioactive molecules in bone and cartilage tissue engineering.” Advanced drug delivery reviews 59.4-5 (2007): 339-359.

Week of 10/1/2018-10/06/2018

Engineering cells for the regeneration of a damaged tissue such as blood vessel or for the eradication of diseases causing agents in tissues such as cancer are central themes in tissue engineering. For this week, using lecture eight, I will discuss how can we potentially re-engineer macrophage activation pathway so that we can specifically deactivate this pathway to regenerate a damaged tissue or activate to cause cell apoptosis in cancer cells.
In order to manipulate the cellular machine inside a cell, we have to transmit a chemical molecule that would alter the mechanism of the cellular machine. To engineer a particular tissue to regenerate, a signal has to be sent to alternate the activation of macrophages for wound repair, tissue fibrosis because activation of macrophages causes the activation of adaptive immune systems such as T-cells and B-cells which leads cell apoptosis in a downstream activation. There are substantial challenges in the way of getting a  drug to target organelle and if we can somehow overcome these barriers we can potentially achieve the desired effects in a cellular and tissue level.

The first issue is that the undesired cellular uptake: that’s  when a drug is circulated in a circulatory system, the broken up peptides on the surface of our engineered nanoparticles that carries our drug will be detected by the immunity system and will be sent for degradation to different parts of the organ depending on the location of detection and the type protein coated on the surface of the nanoparticle. We can overcome this barrier by implementing various strategies.

Figure 1: PEGylation of nanoparticle for the improvement of blood circulation time

One of the strategies to use the mechanism of PEGylation, a process of attaching a PEG molecule covalently or non-covalently onto a nanoparticle to reduce the interaction of the antigenic protein on the surface of the nanoparticle with the immune system cells (1). This molecule reduces the number of protein on the surface of the nanoparticle thus reducing the cellular uptake of the nanoparticle with the drug as shown in figure 1. This mechanism merely opened the gateway of solving  the cellular uptake problem,  and since then,  a more robust and improved version of this mechanism has been developed: one of  which is to use a self-peptide thus the cell will recognize that peptide as  its peptide so that it doesn’t trigger the immune system and thus substantially reduces the cellular uptake of that molecule.

However, the use of self-peptide only solves just one of the drug delivery issue. Another issue is that we want cellular uptake of our drug but still we want to deliver our drug to a target cell and a target organelle in that cell. That’s we have to have a system to overcome the cellular semi-permeability layer barrier because the membrane is semipermeable to the selected molecule: it doesn’t just let in any molecule into the cells. It acts as one of the primary mechanism to protect the cell and regulate the movement of the molecule into and out of the cell using a sophisticated membrane protein integration.

Figure 2: The HIV TAT penetrates the cellular membrane without inducing a cellular damage

 To solve this problem, we need to acquire understanding how viruses such HIV gets into the cell.  The HIV is one of the sophisticated molecule machinery that hijack the human cellular regulations to potentially get into the cell and use the human cellular machinery to make its own necessary protein and other molecules by inserting its DNA into the human DNA. In early 1990,  researchers were able to recognize a sequence of a protein that the virus employed to enter into the cell as shown in figure 2: cell penetrating peptide (CPP) called HIV TAT sequence (2). That is another incredibly amazing achievement by the virus. When I studied about this mechanism, I always amazed how the sophisticated cellular machinery regulation by using proteins and molecular chemistry help sustain life this long against hypermutation cable virus molecule. As the size of a cell or a molecule (such as human cell compare to single bacteria compare to the virus), the speed of evolution increase.  The virus is just a molecule which is considerably small compared to a human cell, thus having the enormous advantage of hypermutation in a short period of time to hijack the cellular regulations. It seems to me that without the human substantial effort in research to the breakthrough in understanding the cellular mechanisms, the speculation of cell surviving in the competition of virus would be difficult just like the HIV epidemic would have killed most of the human species. Thank you for the scientific research and discoveries, now we know a lot about cellar mechanism even though we have to go a lot to specifically modulate the cellular regulations in tissue engineering. Now let’s get back to the idea of learning from the virus, now we can hijack the virus mechanism for our advantage in tissue engineering. Using the peptide sequence that the virus uses and with even a more improved version, we can now send nanoparticle with the drug into the cell without inducing damaging to that cellular membrane.

At this point, the first two major barriers are solved. The next issue is that once the nanoparticle gets into the cell, the cell immediately captures the nanoparticle with liposome for vesicle (pack) and to send it to lysosome to degrade the molecule. Again this regulation has a substantial evolutionary advantage because the cell uses this mechanism to clean up detrimental material in the cell because the cell doesn’t have a recognition what that nanoparticle contains (the nanoparticle gets into the cell without any regulation of the cell and so the nanoparticle has to deal with the fate of degradation).

Figure 3: endosomal escape mechanism to deliver a drug to a target organelle in the cell.

For this barrier, a lot of agents have developed with some side effect and benefits to altering the liposome function before the nanoparticle gets to the lysosome for degradation.  One of the mechanisms we can use for this purpose is the endosomal escape through membrane fusion to release a cargo containing the drug for a target molecule in a cell as shown in figure 3 (3).

Finally, once we passed the last stage, we can engineer our nanoparticle to go into the target organelles such as mitochondria, nucleus…or to exit the cell. For our purpose, at this stage, we can deliver a drug to perform the two ultimate goals. The first goal is to increase tissue regeneration for wound repair via tissue fibrosis by deactivating the macrophage presentation (protosome degradation and presentation). Or the second ultimate goal which is to specifically activate the macrophages in the cancer cell to increase cellular inflammation via T-cells and antibodies to potentially cause cancer cells apoptosis.

Source:

1. Suh, Junghae, et al. “PEGylation of nanoparticles improves their cytoplasmic transport.” International journal of nanomedicine 2.4 (2007): 735.
2. CPP Peptide Synthesis: Make Your Peptide Cell Permeable!www.lifetein.com/Cell_Permeable_Peptides.html.
3. Lönn, Peter, et al. “Enhancing endosomal escape for intracellular delivery of macromolecular biologic therapeutics.” Scientific reports 6 (2016): 32301.

The cardiovascular system is made up of blood vessel networks that supply oxygenated blood to deliver to tissues and to bring back deoxygenated blood back to the heart. This mechanism denotes what keeps an organism keep operating by providing the necessary nutrients and oxygen to tissues. However, when the mechanism is disrupted, it can potentially lead to functionality break down or even to sudden death. Amongst the sever problems, heart attack and stroke are the leading cause of cardiovascular-associated mortality.

Figure 1: ischemic stroke

For this week’s blog, I’ll discuss ischemic stroke and tie it back to lecture 6, that help me to use the materials in class and to propose a potential solution for ischemic stroke by combining tissue engineering and slow release nanoscale polymer drug delivery mechanism.

An ischemic stroke is a condition in which a part of the blood vessel network (carotid artery) that delivers the blood to the brain is interrupted which lead an immediate brain cell death. Storke is a significant issue, just in the US alone over 140,000 people die of stroke each year (3). Additionally, it’s a preeminent cause of serious and long-term disability. It has a high prevalence rate in people over the age of 55. Moreover, ischemic stroke accounts for 88 percent of strokes (2).  Although there are several medical risk factors associated with ischemic stroke, high blood cholesterol, and heart failure problems are the foremost causes of ischemic stroke. Especially, when the blood cholesterol level is high, a plaque is formed up in the carotid arteries which can potentially block blood flow to the brain and causes a stroke (1) as shown in figure 1 above. Cholesterol is a soft lipid fat that is found in the cells or ingested from meals.

As shown in figure 1 above, the build-up of cholesterol blocks the flow of blood and if there is a mechanism to reduce or dissolve that plague, it would be possible to mitigate the ischemic stroke problem in the patients. In fact, there is a drug (glyburide) that can do that or at least try to do that mechanism as shown in Figure 2 below.

Figure 2: Glyburide- a potential drug for lessening ischemic stroke

However, there is a significant drug delivery issue in the brain because the brain’s blood circulatory system is evolved in a unique way to selectively pass substance (called brain blood barrier) to keep out toxins, sustain microenvironment of the brain (such as ion levels) and ultimately to increase immune surveillance with least inflammation. The easy solution for the puzzle may be to surgically deliver a drug that eases the issue. But the risk of performing several brain surgeries to a patient outways the benefits because of pains and potential brain infections. Moreover, in most cases of ischemic stroke patients, the issue is known after a blood vessel is weakened or deteriorated due to a plague or other causes.

So, looking at the problem statement, and the application of biomaterials, can we combine nanoscale slow drug release polymers and tissue engineering to alleviate ischemic stroke? Hopefully! First, a nanoscale drug carrier polymer such PLGA shown in figure 3 below can be used to deliver the glyburide selectively to the brain to release slowly as it degrades slowly (biodegradable).

Figure 3: poly(lactic-co-glycolic acid) (PLGA) structure

The polymer solves the diverse problems of drug delivery barriers associated with a traditional method of drug delivery. First, it eliminates the need for multiple surgeries in a patient. Furthermore, unlike the traditional drug delivery methods, it reduces cell toxification because most of the delivery methods release a high dose of drug in a small site and that toxifies the cells in that area but the PLGA based drug distribution solves that problem by gradual releasing adequate amount of dose for longer period of time, thus lessening toxicity and circumventing  the need to do serval surgeries in a patient’s brain. Moreover, the polymer composition can be engineered to deliver a drug in a targeted tissue at the desired drug release rate. Subsequently, if a carotid artery found to be already damaged, tissue engineering can be incorporated to replace damaged parts of the vessel with soft biomaterials such as polyurethanes to regenerate the damaged parts of the vessel.

Source:

1. Cholesterol and Stroke. (n.d.). Retrieved from www.stroke.org
2. The Internet Stroke Center. (n.d.). Retrieved from http://www.strokecenter.org/patients/about-stroke/stroke-statistics/
3. The Internet Stroke Center. (n.d.). Retrieved from http://www.strokecenter.org/patients/about-stroke/ischemic-stroke/

Welcome to my blog!

I am motivated by the creative mindset and innovation that drives bioengineers to invent new ways of improving human health. At Yale, biomedical engineers are developing biomaterials for use in detecting inflammatory signals which have potential applications in an array of diseases such as diabetes and arthritis. My undergraduate education and research experience in tissue engineering helped me to explore different application of tissue engineering. I am particularly interested in Dr. Anjelica Gonzalez’s work in developing biomaterials. By undertaking additional tissue engineering training in the graduate program, I am well prepared to enter this emerging field with a nuanced understanding of cellular biology, physical engineering, and global health.

In my future blogs, I’m planning on incorporating tools that I learned in the Biomaterials class to discuss their application in tissue engineering and drug delivery for the endovascular system.

Biomaterials for Tissue Engineering

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